Coordination Chemistry of Potentially S,N,Npy-Tridentate Thiosemicarbazones with the {Re(CO)3}+ Fragment and Formation of Hemiaminal Derivatives

Nine potentially S,N,N_py-tridentate thiosemicarbazones (HL) derived from pyridine-2-carbaldehyde or 1-(2-pyridyl)ethanone have been prepared and fully characterized. The X-ray crystal structures of six of them and two hydrochlorides were determined and analyzed. The reaction of the [ReX(CH₃CN)₂(CO)₃]/[ReX(CO)₅] (X = Cl and Br) precursors with these ligands yielded different kinds of compounds: the adducts [ReX(HL)(CO)₃], in which the ligands were S,N-bidentate; the trinuclear species [Re₃Cl₂(L²³)(HL²³)(CO)₉]; and the thiosemicarbazonate compounds [Re(L)(CO)₃], where the ligand is S,N,N_py-tridentate. Besides, the reaction in methanol or ethanol of the thiosemicarbazones derived from aldehydes yielded S,N,N_py-tridentate hemiaminal cationic [Re(HL^OR)(CO)₃]X and neutral [Re(L^OMe)(CO)₃] complexes after the coordinated ligand underwent addition of the alcohol group to the imine bond. The reactivity of the complex [ReX(HL)(CO)₃] in MeOH and NEt₃ led to the formation of dinuclear [Re₂(L)₂(CO)₆], where the thiosemicarbazonate is again S,N-bidentate. The influence that the substituents on the thiosemicarbazone ligands have on the stability of the complexes and the effect of the reaction medium on the resulting compounds have been analyzed.

Crystal structure and magnetic study of the complex salt [RuCp(PTA)2-μ-CN-1κC:2κN-RuCp(PTA)2][Re(NO)Br4(EtOH)0.5(MeOH)0.5]

A new RuII-ReII complex salt, μ-cyanido-κ2 C:N-bis-[(η5-cyclo-penta-dien-yl)bis(3,5,7-tri-aza-phosphaadamantane-κP)ruthenium(II)] tetra-bromido-(ethanol/methanol-κO)nitrosylrhenate(II), [Ru(CN)(C5H5)2(C6H12N3P)4][ReBr4(NO)(CH4O)0.5(C2H6O)0.5] or [RuCp(PTA)2-μ-CN-1κC:2κ2 N-RuCp(PTA)2][Re(NO)Br4(EtOH)0.5(MeOH)0.5] (PTA = 3,5,7-tri-aza-phosphaadaman-tane) was obtained and characterized by single-crystal X-ray diffraction, elemental analysis and infrared spectroscopy. The title salt was obtained by liquid-liquid diffusion of methanol/DMSO solutions of (NBu4)[Re(NO)Br4(EtOH)] and [(PTA)2CpRu-μ-CN-1κC:2κ2 N-RuCp(PTA)2](CF3SO3). The RuII and ReII independent moieties correspond to a binuclear and mononuclear complex ion, respectively. A deep geometrical parameter analysis was performed, and no significant differences were found with earlier reports containing similar mol-ecules. The magnetic properties were investigated in the temperature range 2.0-300 K, and the complex behaves as a quasi-magnetically isolated spin doublet with weak anti-ferromagnetic inter-actions.

Synthesis of Novel Dinuclear N-Substituted 4-(Dimethylamino)benzaldehyde Thiosemicarbazonates of Rhenium(I): Formation of Four- and/or Five-Membered Chelate Rings, Conformational Analysis, and Reactivity

The reaction of fac-[ReX(CH₃CN)₂(CO)₃] (X = Cl, Br) with N-phenyl-[4-(dimethylamino)benzaldehyde] thiosemicarbazone (HL^A) or N-4-methoxybenzyl-[4-(dimethylamino)benzaldehyde] thiosemicarbazone (HL^B) under controlled synthetic conditions gave 4 mononuclear [ReX(HL)(CO)₃] (X = Cl, Br) and 16 dinuclear [Re₂L₂(CO)₆] compounds. These complexes were obtained as single crystals, and their structures were established by X-ray diffraction. The structural study of these dimers showed the formation of several solvates, the presence of linkage isomerism, and the stabilization of four- and/or five-membered chelate rings. The different ligand coordination modes (a new μ-κ²-S,N2:κ-N3 coordination mode for a thiosemicarbazone ligand was observed), the conformation of the thiosemicarbazone chain in each case, the formal symmetry of the dimers, and the role of the synthetic procedure in the stability of the different chelate rings were analyzed and are discussed. Theoretical calculations in the gas phase were performed for the dimers with the HL^A ligand in order to identify the thermodynamically most stable species. The behavior and structural stability of dimers in dimethyl sulfoxide and acetone solutions was investigated by ¹H NMR spectroscopy. The strength of the Re^I-L bond in solution was evidenced by the formation of [Re₂(L^NO₂)₂(CO)₆] and [Re(L^A)(py)(CO)₃] upon reaction of the corresponding dimer with concentrated nitric acid and pyridine, respectively.

Chemical and biological studies of Re(I)/Tc(I) thiosemicarbazonate complexes relevant for the design of radiopharmaceuticals

Five thiosemicarbazones (H₂Lⁿ) derived from 4,6-diacetylresorcinol (n = 1-4) and salicylaldehyde (n = 5) have been synthesized and spectroscopically characterized. Single crystal X-ray diffraction studies on some of them show that the molecular structure is dominated by intramolecular hydrogen bonds involving the O(1)-H group of the resorcinol/salicylaldehyde group and the azomethinic nitrogen atom and sulfur atom of the thiosemicarbazone arm. All of the ligands react with fac-[ReBr(CO)₃(CH₃CN)₂] in the presence of NEt₃ to form the stable anionic complexes [NHEt₃][fac-[Re(Lⁿ)(CO)₃] (1-5). The thiosemicarbazonate ligand, as suggested by spectroscopic data and confirmed by X-ray diffraction, acts as a tridentate S,N,O system. The complexes are stable in solution for weeks, although other dimeric species were also detected by X-ray diffraction analysis. The reaction of fac-[^99mTc(CO)₃(H₂O)₃]⁺ with the appropriate ligand at 100 °C for 30 min yielded the complexes [fac-[^99mTc(Lⁿ)(CO)₃]^- (Tc1-Tc5). The radiochemical yield and purity were determined by HPLC and their chemical identity was ascertained by comparing their radiochromatogram with the chromatogram of the rhenium congeners (1-5). The results of biodistribution studies in mice on the five technetium compounds showed rapid blood clearance and fast liver uptake that slowly cleared into the intestines, a finding that indicates the hepatobiliary tract as the major excretory pathway. HPLC analysis of urine and blood serum samples from mice injected with the ^99mTc complexes confirmed their in vivo stability since the predominant radiochemical species had the same retention time as the corresponding injected compound.

Thiosemicarbazonate complexes with affinity for amyloid-β fibers: synthesis, characterization and biological studies

Aim: Obtain radioimages of amyloid-β fibers using 99mTc-complexes. Methodology: Tridentate thiosemicarbazone and thiocarbonohydrazone ligands containing fragments (stilbene, azobenzene, benzothiazole or benzoxazole) with affinity for amyloid-ß fibers and its Re(I) complexes have been prepared. The molecular structures of several ligands and complexes were determined by x-ray diffraction. Binding affinity studies toward Aß1-42 fibers were performed for the ligands and Re(I) complexes. The ability of formation of some 99mTc(I) complexes, their biodistribution and in vivo stability have been established. Results & conclusion: Complexes of stilbene and benzothiazole thiosemicarbazonates show similar affinity for amyloid-β fibers to the free ligand. These 99mTc complexes present a reasonable in vivo stability and a low capability to cross the blood–brain barrier although not sufficient to brain amyloid imaging.